HGF is a mesenchyme-derived pleiotrophic factor with mitogenic, motogenic and morphogenic activities on a number of different cell types. HGF effects are mediated through a specific tyrosine kinase, c-Met, and aberrant HGF and c-Met expression are frequently observed in a variety of tumors. (Maulik et al., Cytokine & Growth Factor Reviews (2002), 13:41-59; Danilkovitch-Miagkova & Zbar, J. Clin. Invest. (2002), 109(7):863-867). Regulation of the HGF/c-Met signaling pathway is implicated in tumor progression and metastasis. (Trusolino & Comoglio, Nature Rev. (2002), 2:289-300).
HGF binds the extracellular domain of the Met receptor tyrosine kinase (RTK) and regulates diverse biological processes such as cell scattering, proliferation, and survival. HGF-Met signaling is essential for normal embryonic development especially in migration of muscle progenitor cells and development of the liver and nervous system (Bladt et al., Nature 376, 768-771. 1995; Hamanoue et al. J. Neurosci. Res. 43, 554-564. 1996; Schmidt et al., Proc. Natl. Acad. Sci. USA 94, 11445-11450, 1995; Uehara et al., Nature 373, 702-705, 1995). Developmental phenotypes of Met and HGF knockout mice are very similar suggesting that HGF is the cognate ligand for the Met receptor (Schmidt et al., Proc. Natl. Acad. Sci. USA 94, 11445-11450, 1995; Uehara et al., Nature 373, 702-705, 1995). HGF-Met also plays a role in liver regeneration, angiogenesis, and wound healing (Bussolino et al., J. Cell Biol. 119, 629-641 1992; Nusrat et al., J. Clin. Invest. 93, 2056-2065 1994). The precursor Met receptor undergoes proteolytic cleavage into an extracellular subunit and membrane spanning subunit linked by disulfide bonds (Tempest et al., Br. J. Cancer 58, 3-7 1988). The subunit contains the cytoplasmic kinase domain and harbors a multi-substrate docking site at the C-terminus where adapter proteins bind and initiate signaling. Upon HGF binding, activation of Met leads to tyrosine phosphorylation and downstream signaling through Gab1 and Grb2/Sos mediated PI3-kinase and Ras/MAPK activation respectively, which drives cell motility and proliferation (Furge et al., Oncogene 19, 5582-5589 2000; Hartmann et al., J. Biol. Chem. 269, 21936-21939 1994; Ponzetto et al., Cell 87, 531-542 1996; and Royal and Park, J. Biol. Chem. 270, 27780-27787 1995).
Met overexpression or gene-amplification has been observed in a variety of human cancers. For example, Met protein is overexpressed at least 5-fold in colorectal cancers and reported to be gene-amplified in liver metastasis (Di Renzo et al., Clin. Cancer Res. 1, 147-154, 1995; Liu et al., Oncogene 7, 181-185 1992). Met protein is also reported to be overexpressed in oral squamous cell carcinoma, hepatocellular carcinoma, renal cell carcinoma, breast carcinoma, and lung carcinoma (Jin et al., Cancer 79, 749-760 1997; Morello et al., J. Cell Physiol. 189, 285-290 2001; Natali et al., Int. J. Cancer 69, 212-217. 1996; Olivero et al., Br. J. Cancer 74, 1862-1868 1996; Suzuki et al., Hepatology 20, 1231-1236 1994). In addition, overexpression of mRNA has been observed in hepatocellular carcinoma, gastric carcinoma, and colorectal carcinoma (Boix et al., Hepatology 19, 88-91 1994; Kuniyasu et al., Int. J. Cancer 55, 72-75 1993; Liu et al., Oncogene 7, 181-185 1992).
A number of mutations in the kinase domain of Met have been found in renal papillary carcinoma which leads to constitutive receptor activation (Olivero et al., Int. J. Cancer 82, 640-643 1999; Schmidt et al., Nat. Genet. 16, 68-73 1997; Schmidt et al., Oncogene 18, 2343-2350 1999). These activating mutations confer constitutive Met tyrosine phosphorylation and result in MAPK activation, focus formation, and tumorigenesis (Jeffers et al., Proc. Natl. Acad. Sci. USA 94, 11445-11450 1997). In addition, these mutations enhance cell motility and invasion (Giordano et al., 2000; Lorenzato et al., Cancer Res. 62, 7025-7030 2002). HGF-dependent Met activation in transformed cells mediates increased motility, scattering, and migration which eventually leads to invasive tumor growth and metastasis (Jeffers et al., Mol. Cell Biol. 16, 1115-1125 1996; Meiners et al., Oncogene 16, 9-20 1998).
Met is a member of the subfamily of RTKs which include Ron and Sea (Maulik et al., Cytokine Growth Factor Rev. 13, 41-59 2002). Prediction of the extracellular domain structure of Met suggests shared homology with the semaphorins and plexins. The N-terminus of Met contains a Sema domain of approximately 500 amino acids that is conserved in all semaphorins and plexins. The semaphorins and plexins belong to a large family of secreted and membrane-bound proteins first described for their role in neural development (Van Vactor and Lorenz, Curr. Biol. 9, R201-204 1999). However, semaphorin overexpression has been correlated with tumor invasion and metastasis. A cysteine-rich PSI domain (also referred to as a Met Related Sequence domain) found in plexins, semaphorins, and integrins lies adjacent to the Sema domain followed by four IPT repeats that are immunoglobulin-like regions found in plexins and transcription factors. A recent study suggests that the Met Sema domain is sufficient for HGF and heparin binding (Gherardi et al., (2003). Functional map and domain structure of Met, the product of the c-Met protooncogene and receptor for hepatocyte growth factor/scatter factor. Proc. Natl. Acad. Sci. USA 2003). Furthermore, Kong-Beltran et al. (Cancer Cell (2004), 6:61-73) have reported that the Sema domain of Met is necessary for receptor dimerization and activation.
Numerous molecules targeted at the HGF/c-Met pathway have been reported. These molecules include antibodies such as those described in U.S. Pat. No. 5,686,292. A portion of the extracellular domain of c-Met has also been shown to be capable of antagonistic effects against the HGF/c-Met pathway. In view of the important role that this pathway plays in the etiology of various pathological conditions, however, it is clear that there continues to be a need for agents that have clinical attributes that are optimal for development as therapeutic agents.